For the preparation of certain paper grades, there is a need for counteracting or inhibiting the natural liquid-absorbing properties of paper. Examples of such paper grades are writing paper and printing paper. Other examples are board or paperboard intended for juice and milk cartons. Still another example is photobase paper.
Paper grades such as the aforementioned require liquid-repellent properties. There are many different methods available for achieving liquid repellancy, (i.e. hydrophobicity or sizing). One of these is to add during the paper manufacturing process an emulsion of a hydrophobic material. Many different hydrophobic materials can be utilized. Among the most effective are the so called hydrophobic cellulose-reactive sizing agents. It is believed that when using this type of agent, sizing is obtained by a reaction between the hydrophobic material and the hydroxyl groups of the cellulose. Examples of typical hydrophobic sizing agents are alkylketene dimers, alkenyl succinic anhydrides and fatty isocyanates.
Since the hydrophobic sizing agents are insoluble in water they are employed in the paper manufacturing process in the form of an emulsion. As emulsifiers surfactants may be utilized, but in general surfactants give emulsions having a poor effectiveness as they show a low affinity to the cellulose fiber, which in turn means that much of the hydrophobic sizing agent will be lost when dewatering the paper stock. It has been found that cationic polymers are comparatively better emulsifying agents. Examples of cationic polymers employed for this purpose are described in U.S. Pat. No. 3,130,118 which discloses the use of a cationic starch as an emulsifying agent, and U.S. Pat. No. 4,240,935 which emphasizes the advantages of using as emulsifying agents resins comprising the reaction product of epichlorohydrin and an aminopolyamide prepared from adipic acid and diethylene triamine.
In hydrophobic sizing compositions, the cationic polymer desirably fulfills many functions. Firstly it should stabilize the emulsions. Secondly it should enhance retention of the hydrophobic or sizing agent, either alone or in combination with a separately added retention agent on the paper. Furthermore, the choice of emulsifier may influence the degree of sizing so as to enable the manufacture of a more hydrophobic paper. U.S. Pat. No. 4,382,129 discloses a cationic polymer having this property. Moreover, it has been found that certain cationic polymers may enhance the rate of sizing which is developed over time with the cellulose-reactive sizing agents. U.S. Pat. No. 4,317,756 discloses polymers having such an effect.
For each of these different effects it is very difficult or unreliable to predict how a given cationic polymer will act or work and in general this is completely impossible, as it has not been shown or proved how the choice of cationic polymer affects the total efficiency of the combination of hydrophobic cellulose-reactive sizing agent and cationic polymer. In spite of the fact that hydrophobic cellulose-reactive sizing agents have been available on the market for more than 20 years and that during said years the products have been considerably improved, there are still improvements to be achieved in this art. Comparatively large amounts of cellulose-reactive sizing agents have to be used to obtain the desired liquid repellancy with the hydrophobic sizing compositions of the prior art. A reduction of the amount of sizing agent to be used to obtain the necessary degree of sizing would mean great savings in material costs. In addition, hydrophobic cellulose-reactive sizing agents do not give an immediate sizing. Such an action can be accelerated by the use of a combination of certain types of cationic polymers as have been previously described, but unfortunately these strongly cationic polymers have the disadvantage that they considerably impair the effectiveness of optical brighteners used to improve the whiteness of the paper, resulting in increased consumption of optical brighteners. This sets limits to the machine speed for certain high whiteness grades of paper as a certain minimum degree of sizing must have been obtained when the paper passes the size press or an on-line coating unit, as otherwise the paper will be very weak and will easily rupture. For certain paper grades, it would be desirable to reach a higher degree of sizing than is technically obtainable today. This applies for instance to milk and juice paper boards and to photobase papers.